1. Field of the Invention.
The present invention relates to apparatus for use in dimming fluorescent lamps and, more particularly, to a high efficiency circuit having a large dimming range ratio suitable for use in applications such as flat panel displays where ambient light may change from very dim to very bright as, for example, in an aircraft environment.
2. Description of the Prior Art.
Fluorescent lamp dimming circuits for use in general area lighting are well known in the prior art. For example, co-pending applications Serial No. 39,111 entitled "Time Delay Initialization Circuit", Serial No. 239,193 entitled "Notch Cutting Circuit with Minimal Power Dissipation", and Serial No. 39,209 entitled "Power Control Circuit for Inductive Loads", all of which were filed Aug. 31, 1988, and are assigned to the assignee of the present invention, show a fluorescent lamp dimming system in which the alternating signal supplying the power to the lamp is cut with a notch of variable width so as to reduce the power applied to the lamp and thereby provide the desired dimming.
While such circuits are useful for general area lighting where the ratio between the brightest and the dimmest is not very large, in some applications there is a need for fluorescent light dimming in which the dimming ratio is desired to be greater than, for example, 10,000:1. Such an application is found in aircraft, and especially military aircraft, where display systems in the form of color liquid crystal flat panels are used. These displays need a back lighting system to make information visible to the pilot under ambient lighting conditions that may go from near blackness at night to extreme bright glare facing into the sun. Since it is also desired that the back lighting color not change over the dimming range, fluorescent lights are preferred since their color is not altered by dimming but rather by the selection of the appropriate composition of phosphorus coating within the lamp. Accordingly, the brightness of the fluorescent lamp needs to vary by a large amount in order for the pilot to be able to view the display under all ambient light conditions. It is also desirable that the change be rather exponential, because under dark conditions the variation in luminance necessary to accommodate changes in ambient light are far less than the variation in luminance necessary under very bright conditions. Accordingly, it is desired that control of the output of the fluorescent lamps vary the brightness of the lamps by a relatively small amount under dark ambient light conditions, but by a relatively large amount under bright sunlight conditions and a log or exponential response is preferred. The system should also be free of swirls, flicker and discontinuities, be capable of withstanding temperatures from about -55.degree. C. to +75.degree. C. with a smooth response to the pilot's dimming command, and be able to provide a large number of cold starts and hours of operation while maintaining a high circuit efficiency. In some cases, the dimmer needs to drive multiple lamps (for reliability) in parallel and, accordingly, electrical isolation is needed so that if a lamp fails it will not affect the luminance output of the remaining lamps.
While some companies have provided fluorescent backlight assemblies, to date the best dimming range is no greater than 1,000:1, the efficiency of such lamps is believed to be only around 50% and such lamps are believed to have some luminescence instabilities.